Animation Nodes : how can I switch between different setups?
Animation Nodes Object Container

The Animation Nodes Object Container is created as a helper object for the Object Instancer node, in other words, it is only created when the Object Instancer node is used. Notice that it doesn't get removed when the Object Instancer node is removed and it is a single object even if you have multiple Object Instancer nodes.

Node Trees

If multiple Node Trees are presents, all of them executes based on the defined Auto Execution settings. So if both have Always option enabled, both will execute simultaneously. So there really isn't an "Active Node Tree". Subprograms are shared across Node Trees, so you can call a subprogram defined in one node tree in another.

Animation Nodes provide a fast way to disable/enable Auto Execution for the available node trees, this is done through the Overview Panel: